Dynamic linking

Do your installed programs share dynamic libraries?

Findings: not really

Over half of your libraries are used by fewer than 0.1% of your executables. Number of times each dynamic library is required by a program

libs.awk

/\t.*\.so.*/ {
	n=split($1, p, "/")
	split(p[n], l, ".")
	lib=l[1]
	if (libs[lib] == "") {
		libs[lib] = 0
	}
	libs[lib] += 1
}
END {
	for (lib in libs) {
		print libs[lib] "\t" lib
	}
}

Usage

$ find /usr/bin -type f -executable -print \
  | xargs ldd 2>/dev/null \
  | awk -f libs.awk \
  | sort -rn > results.txt
$ awk '{ print NR "\t" $1 }' < results.txt > nresults.txt
$ gnuplot
gnuplot> plot 'nresults.txt'

my results

$ find /usr/bin -type f -executable -print | wc -l
5688
$ head -n20 < results.txt
4496	libc
4484	linux-vdso
4483	ld-linux-x86-64
2654	libm
2301	libdl
2216	libpthread
1419	libgcc_s
1301	libz
1144	libstdc++
805	liblzma
785	librt
771	libXdmcp
771	libxcb
771	libXau
755	libX11
703	libpcre
667	libglib-2
658	libffi
578	libresolv
559	libXext

Is loading dynamically linked programs faster?

Findings: definitely not

ex.c

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
	struct timespec ts;
	clock_gettime(CLOCK_MONOTONIC, &ts);
	fprintf(stdout, "%ld\t", ts.tv_nsec);
	fflush(stdout);
	if (argc == 1) {
		char *args[] = { "", "", NULL };
		execvp(argv[0], args);
	} else {
		fprintf(stdout, "\n");
	}
	return 0;
}

test.sh

#!/bin/sh
i=0
while [ $i -lt 1000 ]
do
	./ex
	i=$((i+1))
done

My results

$ musl-gcc -o ex ex.c
$ ./test.sh | awk 'BEGIN { sum = 0 } { sum += $2-$1 } END { print sum / NR }'
137263
$ musl-gcc -static -o ex ex.c
$ ./test.sh | awk 'BEGIN { sum = 0 } { sum += $2-$1 } END { print sum / NR }'
64048

Wouldn't statically linked executables be huge?

Findings: not really

On average, dynamically linked executables use only 4.6% of the symbols on offer from their dependencies. A good linker will remove unused symbols. % of symbols requested by dynamically linked programs from the libraries that it depends on

nsyms.go

package main

import (
	"bufio"
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
)

func main() {
	ldd := exec.Command("ldd", os.Args[1])
	rc, err := ldd.StdoutPipe()
	if err != nil {
		panic(err)
	}
	ldd.Start()

	var libpaths []string
	scan := bufio.NewScanner(rc)
	for scan.Scan() {
		line := scan.Text()[1:] /* \t */
		sp := strings.Split(line, " ")
		var lib string
		if strings.Contains(line, "=>") {
			lib = sp[2]
		} else {
			lib = sp[0]
		}
		if !filepath.IsAbs(lib) {
			lib = "/usr/lib/" + lib
		}
		libpaths = append(libpaths, lib)
	}
	ldd.Wait()
	rc.Close()

	syms := make(map[string]interface{})
	for _, path := range libpaths {
		objdump := exec.Command("objdump", "-T", path)
		rc, err := objdump.StdoutPipe()
		if err != nil {
			panic(err)
		}
		objdump.Start()
		scan := bufio.NewScanner(rc)
		for i := 0; scan.Scan(); i++ {
			if i < 4 {
				continue
			}
			line := scan.Text()
			sp := strings.Split(line, " ")
			if len(sp) < 5 {
				continue
			}
			sym := sp[len(sp)-1]
			syms[sym] = nil
		}
		objdump.Wait()
		rc.Close()
	}

	objdump := exec.Command("objdump", "-R", os.Args[1])
	rc, err = objdump.StdoutPipe()
	if err != nil {
		panic(err)
	}
	objdump.Start()
	used := make(map[string]interface{})
	scan = bufio.NewScanner(rc)
	for i := 0; scan.Scan(); i++ {
		if i < 5 {
			continue
		}
		sp := strings.Split(scan.Text(), " ")
		if len(sp) < 3 {
			continue
		}
		sym := sp[len(sp)-1]
		used[sym] = nil
	}
	objdump.Wait()
	rc.Close()

	if len(syms) != 0 && len(used) != 0 && len(used) <= len(syms) {
		fmt.Printf("%50s\t%d\t%d\t%f\n", os.Args[1], len(syms), len(used),
			float64(len(used)) / float64(len(syms)))
	}
}

Usage

$ find /usr/bin -type f -executable -print | xargs -n1 ./nsyms > results.txt
$ awk '{ n += $4 } END { print n / NR }' < results.txt

my results

Will security vulnerabilities in libraries that have been statically linked cause large or unmanagable updates?

Findings: not really

Not including libc, the only libraries which had "critical" or "high" severity vulnerabilities in 2019 which affected over 100 binaries on my system were dbus, gnutls, cairo, libssh2, and curl. 265 binaries were affected by the rest.

The total download cost to upgrade all binaries on my system which were affected by these CVEs in 2019 is 3.8 GiB. This is reduced to 1.0 GiB if you eliminate glibc.

It is also unknown if any of these vulnerabilities would have been introduced after the last build date for a given statically linked binary; if so that binary would not need to be updated. Many vulnerabilities are also limited to a specific code path or use-case, and binaries which do not invoke that code path in their dependencies will not be affected. A process to ascertain this information in the wake of a vulnerability could be automated.

arch-security

extractcves.py

import email.utils
import mailbox
import re
import shlex
import time

pacman_re = re.compile(r'pacman -Syu .*')
severity_re = re.compile(r'Severity: (.*)')

mbox = mailbox.mbox("arch-security.mbox")
for m in mbox.items():
    m = m[1]
    date = m["Date"]
    for part in m.walk():
        if part.is_multipart():
            continue
        content_type = part.get_content_type()
        [charset] = part.get_charsets("utf-8")
        if content_type == 'text/plain':
            body = part.get_payload(decode=True).decode(charset)
            break
    pkgs = pacman_re.findall(body)
    severity = severity_re.findall(body)
    date = email.utils.parsedate(date)
    if len(pkgs) == 0 or date is None:
        continue
    if date[0] <= 2018 or date[0] > 2019:
        continue
    severity = severity[0]
    args = shlex.split(pkgs[0])
    pkg = args[2].split(">=")[0]
    print(pkg, severity)

$ python3 extractcves.py | grep Critical > cves.txt
$ xargs pacman -Ql < cves.txt | grep \\.so | awk '{print $1}' | sort -u>affected.txt
# Manually remove packages like Firefox, Thunderbird, etc; write remainder.txt
$ xargs pacman -Ql < remainder.txt | grep '/usr/lib/.*.so$' | awk '{ print $2 }' > libs.txt
$ ldd /usr/bin/* >ldd.txt
$ ./scope.sh <libs.txt | sort -nr >sobjects.txt

sobjects.txt is a sorted list of shared objects and the number of executables that link to them. To find the total size of affected binaries, I ran the following command:

# With libc
$ egrep -la 'libc.so|libm.so|libdl.so|libpthread.so|librt.so|libresolv.so|libdbus-1.so|libgnutls.so|libcairo.so|libutil.so|libssh2.so|libcurl.so|libcairo-gobject.so|libcrypt.so|libspice-server.so|libarchive.so|libSDL2-2.0.so|libmvec.so|libmagic.so|libtextstyle.so|libgettextlib-0.20.2.so|libgettextsrc-0.20.2.so|libMagickWand-7.Q16HDRI.so|libMagickCore-7.Q16HDRI.so|libbfd-2.34.0.so|libpolkit-gobject-1.so|libwebkit2gtk-4.0.so|libjavascriptcoregtk-4.0.so|libpolkit-agent-1.so|libgs.so|libctf.so|libSDL.so|libopcodes-2.34.0.so|libQt5WebEngine.so|libQt5WebEngineCore.so|libctf-nobfd.so|libcairo-script-interpreter.so' /usr/bin/* | xargs wc -c
# Without libc
$ egrep -la 'libdbus-1.so|libgnutls.so|libcairo.so|libssh2.so|libcurl.so|libcairo-gobject.so|libcrypt.so|libspice-server.so|libarchive.so|libSDL2-2.0.so|libmvec.so|libmagic.so|libtextstyle.so|libgettextlib-0.20.2.so|libgettextsrc-0.20.2.so|libMagickWand-7.Q16HDRI.so|libMagickCore-7.Q16HDRI.so|libbfd-2.34.0.so|libpolkit-gobject-1.so|libwebkit2gtk-4.0.so|libjavascriptcoregtk-4.0.so|libpolkit-agent-1.so|libgs.so|libctf.so|libSDL.so|libopcodes-2.34.0.so|libQt5WebEngine.so|libQt5WebEngineCore.so|libctf-nobfd.so|libcairo-script-interpreter.so' /usr/bin/* | xargs wc -c

Test environment

• Arch Linux, up-to-date as of 2020-06-25
• 2188 packages installed
• gcc 10.1.0